1. Field of the Invention
This invention relates generally to the dispersing of liquids into fluidized solids. More specifically, this invention relates to a method and apparatus for dispersing a hydrocarbon feed into a transversely flowing stream of fluidized catalyst particles.
2. Description of the Prior Art
There are a number of continuous cyclical processes employing fluidized solid techniques in which carbonaceous materials are deposited on the solids in a contacting zone and the solids are conveyed during the course of the cycle to another zone where carbon deposits are at least partially removed by combustion in an oxygen-containing medium. The solids from the latter zone are subsequently withdrawn and reintroduced in whole or in part to the contacting zone.
One of the more important processes of this nature is the fluid catalytic cracking (FCC) process for the conversion of relatively high-boiling hydrocarbons to lighter hydrocarbons boiling in the heating oil or gasoline (or lighter) range. The hydrocarbon feed is contacted in one or more reaction zones with the particulate cracking catalyst maintained in a fluidized state under conditions suitable for the conversion of hydrocarbons.
It has been found that the method of contacting the feedstock with the solids can dramatically affect the performance of the contacting zone. Ideally, the feed is instantaneously dispersed as it enters the riser over the entire cross-section of a stream of solids that is moving up the riser. A complete and instantaneous dispersal of feed across the entire cross-section of the riser is not possible, but good results have been obtained by injecting a highly atomized feed into a pre-accelerated stream of particles. However, the dispersing of the feed throughout the particles takes some time so that there is some non-uniform contact between the feed and particles as previously described. Non-uniform contacting of the feed and the particles, for the time it is in the axial contact zone, exposes portions of the feed to the particles for longer periods of time which in turn can produce overcracking and reduce the quality of reaction products.
It has been a long recognized objective in the FCC process to maximize the dispersal of the hydrocarbon feed into the particulate catalyst suspension. Dividing the feed into small droplets improves dispersion of the feed by increasing the interaction between the liquid and solid. It is well known that agitation or shearing can atomize a liquid hydrocarbon feed into fine droplets which are then directed at the fluidized solid particles. It is believed that as droplet sizes become small enough, they can completely vaporize before contacting the solids. However, it is also known that the improvements in dispersal and vaporization must be balanced against the smaller momentum and decrease in penetration across the catalyst stream that results as feed droplets become smaller. A variety of methods are known for shearing such liquid streams into fine droplets.
U.S. Pat. No. 3,071,540 (McMahon et al) discloses a feed injection apparatus for an FCC unit wherein a high velocity stream of gas, in this case steam, converges around the stream of oil upstream of an orifice through which the mixture of steam and oil is discharged. Initial impact of the steam with. the oil stream and subsequent discharge through the orifice atomizes the liquid oil into a dispersion of fine droplets which contact a stream of coaxially flowing catalyst particles. U.S. Pat. No. 4,434,049 (Dean et al) shows a device for injecting a fine dispersion of oil droplets into a fluidized catalyst stream wherein the oil is first discharged through an orifice onto an impact surface located within a mixing tube. The mixing tube delivers a cross-flow of steam which simultaneously contacts the liquid. The combined flow of oil and steam exits the conduit through an orifice which atomizes the feed into a dispersion of fine droplets and directs the dispersion into a stream of flowing catalyst particles. The injection devices of the ""540 and ""049 patents rely on relatively high fluid velocities and pressure drops to achieve atomization of the oil into fine droplets. Providing this higher pressure drop burdens the design and increases the cost of equipment such as pumps and exchangers that are typically used to supply liquid and gas to the feed injection device. The need to replace such equipment may greatly increase the cost of retrofitting an existing liquid-solid contacting installation with such an injection apparatus.
Another useful feature for dispersing feed in FCC units is the use of a lift gas to pre-accelerate the catalyst particles before contact with the feed. Modern FCC units use a pipe reactor in the form of a large, usually vertical, riser in which a gaseous medium upwardly transports the catalyst in a fluidized state. Catalyst particles first enter the riser with zero or negative velocity in the ultimate direction of riser flow. Initiating or changing the direction of particle flow creates turbulent conditions at the bottom of the riser. When feed is introduced into the bottom of the riser, the turbulence can cause mal-distribution and variation in the contact time between the catalyst and the feed. In order to obtain a more uniform dispersion, the catalyst particles are first contacted with a lift gas to initiate upward movement of the catalyst. The lift gas creates a catalyst pre-acceleration zone that moves the catalyst along the riser before it contacts the feed. After the catalyst is moving up the riser, it is contacted with the feed by injecting the feed into a downstream section of the riser. Injecting the feed into a flowing stream of catalyst avoids the turbulence and backmixing of particles and feed that occurs when the feed contacts the catalyst in the bottom of the riser. A good example of the use of lift gas in an FCC riser can be found in U.S. Pat. No. 4,479,870 (Hammershaimb et al).
There are additional references which show the use of a lift gas in non-catalytic systems. For example, in U.S. Pat. No. 4,427,538 (Bartholic), a gas which may be a light hydrocarbon is mixed with an inert solid at the bottom of a vertically confined conduit, and a heavy petroleum fraction is introduced at a point downstream so as to vary the residence time of the petroleum fraction in the conduit. Similarly, in U.S. Pat. No. 4,427,539 (Busch et al), a C4-minus gas is used to accompany particles of little activity up a riser upstream of charged residual oil so as to aid in dispersing the oil.
The orientation of feed injection has also received attention. U.S. Pat. No. 5,139,748 (Lomas et al) shows the use of radially directed feed injection nozzles to introduce feed into an FCC riser. The nozzles are arranged in a circumferential band about the riser and inject feed toward the center of the riser. The nozzle arrangement and geometry of the riser maintain a substantially open riser cross-section over the feed injection and downstream riser sections. Feed atomization, lift gas, and radial injection of feed have been used to more uniformly disperse feed over the cross-section of a riser reaction zone. Nevertheless, as feed contacts the hot catalyst, cracking and volumetric expansion of the hydrocarbons causes an increase in the volumetric rate of fluids passing up the riser. A large portion of this volumetric increase occurs immediately downstream of the feed injection point. Previous feed distributors have allowed this volumetric expansion to occur in a relatively uncontrolled fashion. The uncontrolled volumetric expansion occurring simultaneously with mixing of catalyst and hydrocarbon feed results in mal-distribution that adversely effects the quantity and quality of the products obtained from the cracking reaction. This mal-distribution is caused by turbulent backmixing as well as quiescent zones in the riser section immediately downstream of the feed injection point.
A number of other references also disclose different arrangements for injecting feed radially into a transversely flowing stream of catalyst that is passing through a riser. These methods and apparatus have in common the use of a flow restriction or an orifice arrangement that can provide a venturi effect. Examples of such arrangements are shown in U.S. Pat. No. 5,358,632 (Hedrick), U.S. Pat. No. 5,338,438 (Demoulin et al), U.S. Pat. No. 5,205,992 (van Ommen et al), U.S. Pat. No. 5,552,119 (Holmes), and U.S. Pat. No. 5,562,818 (Hedrick). These patents all show the radial injection of feed immediately at or downstream of the choke point formed by the restriction or venturi orifice arrangement. These methods and apparatus have the advantage of eliminating, to some degree, turbulence associated with the injection of feed and the rapid expansion of the feed as it contacts the hot catalyst. All of these arrangements, however, suffer from at least one defect associated with their continued use in a commercial process. Some of the apparatus are complicated and difficult to fabricate or install. There is also the problem of maintenance of the apparatus for radially dispersing the flow. High particle velocities create a very erosive environment and are likely to cause rapid erosion of any unprotected metals surfaces and high erosion of surfaces even with abrasion resistant coverings. Therefore, use of such venturi arrangements will require designs that inherently protect the nozzles as well as allowing for easy replacement and maintenance of the apparatus. Finally, there is also the need to provide a system that is flexible and easily modified.
It is an object of this invention to provide a method and apparatus that simplifies the reducing or eliminating of non-uniformity in the mixing of particles and feed in the injection of feed into a transversely flowing particle stream. It is a further object of this invention to provide an apparatus and method for injecting feed across a restrict opening into a transverse flowing stream of catalyst. It is a further object of this invention to provide a method and apparatus that is susceptible to simple repair, replacement, or modification that provides an injection or feed across a confined or restricted opening into a transversely flowing stream of catalyst.
The objectives of this invention are achieved by a specific form of a feed injection arrangement that injects feed transversely from the sides of a restricted opening into a stream of flowing particles. The restricted opening is formed by opposing sides that extend as parallel chords on the sides of the riser or other downflow contacting conduit. The restricted opening is preferably formed to create an abrupt restriction that has a venturi effect on the particle stream as it passes into contact with the riser. The parallel sides may be built out with refractory lining or with metal baffles overlayed with abrasion resistant lining, or by parallel pipes that tangentially straddle opposite sides of the catalyst conduit. In more specific forms the invention can use horizontally extended pipes that are tangentially positioned apart from each other across a restricted opening of the conduit that transports the particles. The pipes horizontally inject the feed into the flowing particle stream. Such a pipe arrangement has the advantage of being easily inserted and withdrawn from the riser or other conduit to provide maintenance of the nozzles in the highly erosive environment of the restricted opening. Easy insertion or replacement of the feed pipes also facilitate modification of the nozzles to alter the fluid flow properties of the injected feed.
The restricted opening does provide a venturi effect and actually comprises a quasi-inverted venturi. It is a quasi venturi since it is shaped from gradually sloped, flat surfaces on two sides which alters the catalyst flow path from the typically round conduit cross-section to a confined rectangular orifice where the raw oil is injected. This arrangement has the further advantages of minimizing pressure drop and reducing abrasion at the inlet surface that directs the flow. The downstream outlet from the restricted opening is designed to permit rapid expansion of the hydrocarbon with minimum abrasion of the surfaces above the feed system.
Additional modified forms of the invention include specific configurations of the feed injection pipes. One arrangement of the feed injection pipes will inject a single stream which will usually be all oil. The feed injection pipe may have an arrangement that provides internal baffles for providing cooling of the outer pipe wall to prevent coking therein and an even distribution of the feed to all of the nozzles in both horizontally extended pipe injectors. The feed pipes may also be arranged to contain additional injection nozzles for combining a separate stream such as steam or other atomization medium together with the oil feed at the injection points.
Other mechanical and operational advantages can result from the incorporation of this invention. Such advantages include locating the feed injection closer to the point at which regenerated catalyst initially enters a riser or a downcomer conduit by relying on the restricted opening to provide the dispersion of the particulate material as it is contacted by feed. This eliminates the need for a long pre-acceleration zone to increase the velocity of the particles before it contacts the feed. Moving the feed injection point closer to the point of regenerated catalyst entry can reduce overall height for the process unit. A related process advantage associated with the distribution of particles to the restricted opening provides good catalyst distribution with a higher catalyst density below the initial feed entry point. Accordingly, higher densities reduce the amount of lift gas that may be needed to pre-accelerate catalyst. An additional mechanical advantage of the above arrangement follows from reduced clearance requirements for the piping on the external part of the riser or downcomer conduit as a result of supplying feed in two simple horizontal lines across the restricted opening. As a result, there is no need for complex piping to feed multiple feed distributors located around the circumference of the catalyst-conveying conduit.
Accordingly, in one embodiment, this invention is a method of mixing fluidized particles with an at least partially liquid feedstream comprising hydrocarbons. The method introduces fluidized particles into an upstream section of a particle transport conduit and passes the particles downstream through the transport conduit into a feed contacting zone having a reduced cross-sectional area. A pair of parallel solid chords extend inwardly from the sidewalls of the transport conduit to form the feed contacting zone. The process injects the feed into the particle stream at or immediately downstream of the chords and along a transverse distance that is substantially equal to the length of the chords. The mixture of feed and particles accelerates in a downstream direction in the transport conduit through an acceleration zone that has a continuously increasing cross-sectional area. The mixture of feed and particles passes from the acceleration zone into a section of the transport conduit having a uniform cross-sectional area for further reaction or separation of the feed in the presence of the particles.
In an apparatus embodiment, this invention comprises an elongated transport conduit having an upstream and a downstream end, means for adding fluidized particles to the upstream end, and means for recovering a mixture of particles and feed from the downstream end. Two opposing parallel chord members located transverse to the primary axis of the conduit and separating the upstream and downstream ends of the transport conduit extend inwardly from the sidewall of the transport conduit and tangentially across the transport conduit to define a restricted opening in the center of the conduit. A feed injection line at or adjacent to and parallel with each chord injects feed into the conduit from two sides over a length substantially equal to the length of the chord members.
Additional objections, embodiments, and details of this invention can be obtained from the following xe2x80x9cdetailed descriptionxe2x80x9d.